1. Technical Field
The present invention relates to a liquid ejecting head that ejects liquid by driving a piezoelectric element, and to a liquid ejecting apparatus including the liquid ejecting head. In particular, the invention relates to a liquid ejecting head that ejects liquid from a nozzle by displacing a displacement portion that defines a portion of a pressure chamber by driving a piezoelectric element, and to a liquid ejecting apparatus.
2. Related Art
A liquid ejecting apparatus includes a liquid ejecting head and ejects various types of liquid from the liquid ejecting head. Such liquid ejecting apparatuses include, for example, image recording apparatuses, such as, ink jet printers, ink jet plotters, etc., and have also recently been applied to various kinds of production apparatuses by making use of the feature that enables the apparatus to accurately deposit very small amounts of liquid at predetermined positions. Examples of use of liquid ejecting apparatuses in production apparatuses include a display production apparatus for producing a color filter for a liquid crystal display or the like, an electrode forming apparatus for forming electrodes of an organic electro-luminescence (EL) display, a surface emitting display (SED), etc., and a chip production apparatus for producing a bio-chip (biochemical device). While a recording head for an image recording apparatus ejects liquid inks, a color material ejecting head for a display producing apparatus ejects solutions of color materials of red (R), green (G), and blue (B). Furthermore, an electrode material ejecting head for an electrode forming apparatus ejects an electrode material in a liquid state, and a bioorganic material ejecting head for a chip production apparatus ejects a solution of a bioorganic material.
The aforementioned liquid ejecting heads are constructed to introduce a liquid into a pressure chamber, and eject the liquid from a nozzle communicating with the pressure chamber by causing pressure fluctuation in the liquid in the pressure chamber. The space that forms the aforementioned pressure chamber is formed with high dimensional accuracy by performing anisotropic etching on a crystalline substrate of silicon or the like. Furthermore, a piezoelectric element is preferably used as a pressure generator that causes pressure fluctuation in the liquid in the pressure chamber. The piezoelectric elements vary in terms of configuration. For example, a piezoelectric element has a configuration in which a lower electrode film at a side nearer to the pressure chamber, a piezoelectric body layer of lead zirconium titanate (PZT), etc., and an upper electrode film are stacked by using a film formation technology (see, e.g., JP-A-2007-118193). One of the upper electrode and the lower electrode functions as individual electrodes that are provided individually for each of a plurality of pressure chambers, and the other electrode functions as a common electrode that is common to the pressure chambers. In a piezoelectric body film, portions sandwiched between the upper and lower electrodes are active portions that deform when voltage is applied between the upper and lower electrodes, and portions not sandwiched between the electrodes, that is, portions apart from both or one of the upper and lower electrodes, are non-active portions that do not deform when voltage is applied between the electrodes. An opening portion of each pressure chamber that is formed in a side thereof (the opposite side of the pressure chamber to a nozzle surface side) is closed by an elastic film that is made of, for example, SiO2, and that has flexibility. A piezoelectric element is formed on the elastic film, with an insulation film (made of, e.g., ZrO2) provided therebetween. The elastic film and the insulation film function as a vibration plate.
An evaluation index of the performance of a liquid ejecting head as described above is an index termed expelled volume. Expelled volume means the amount of change in the capacity of a pressure chamber (the volume of liquid expelled from the pressure chamber) that occurs when the piezoelectric element is driven by applying a predetermined drive voltage. By increasing the expelled volume, the liquid can be more efficiently ejected from the nozzle. The approximate size of the expelled volume can be found by multiplying the area of an upper opening of the space that forms a pressure chamber and that is at the opposite side of the space to the nozzle (alternatively, the area of a portion of the vibration plate that tightly closes the upper opening of the space, the portion being capable of being displaced according to the driving of the piezoelectric element (hereinafter, referred to as “displacement portion”, as appropriate)) by the amount of displacement (stroke) of the piezoelectric element that occurs when a predetermined drive voltage is applied. In the case of a relatively large liquid ejecting head (e.g., a liquid ejecting head whose nozzle formation pitch (center-to-center distance between adjacent nozzles) is 1/180 inch or greater, a relatively large capacity of each of the pressure chambers and a relatively large area of each of the upper openings can be secured, so that, accordingly, a relatively large expelled volume can be secured. On the other hand, in a small liquid ejecting head with an increased density of nozzles (e.g., a liquid ejecting head whose nozzle opening pitch is 1/300 or less), the width of each pressure chamber (a dimension thereof in the direction in which pressure chambers are juxtaposed) is smaller than in a large liquid ejecting head. Therefore, with regard to such small liquid ejecting heads, increasing the length of each pressure chamber (a dimension thereof in a direction orthogonal to the pressure chamber juxtaposition direction) is conceivable in order to secure a larger capacity of each pressure chamber and a larger area of each upper opening.
However, if the ratio of the length to the width of each pressure chamber is excessively high, the ease of movement of each displacement portion is impeded, so that the expelled volume deteriorates. Furthermore, there is a problem that the greater the length of the pressure chambers, the greater the dimensions of the liquid ejecting head in planar directions (directions parallel to the nozzle surface). Moreover, even if the amount of displacement of the piezoelectric elements is increased by contriving an electrode structure of the piezoelectric elements or a piezoelectric body structure, there remains a problem that if the displacement portions are not easily movable, the capability of the piezoelectric elements is not fully utilized.